This invention relates generally to multivibrators, and, more particularly, to a voltage-controlled sweep multivibrator which produces an output waveshape which is an essentially exponential function with respect to time.
A multivibrator is a form of relaxation oscillator consisting normally of two or more active devices, such as transistors or vacuum tubes, interconnected by electric networks. In a multivibrator a portion of the output voltage or current of each active device is applied to the input of the other with such magnitude and polarity as to maintain the devices alternately conducting over controllable periods of time. Such an electronic circuit has an extremely broad range of applications, in such equipment as oscilloscopes, telemetry systems, computer terminals, television cameras and signal processors, electronic music systems, etc.
In the past, multivibrators had an extremely limited range of sweep repetition rates (SRR). This problem was overcome by the utilization of a digital device and associated circuitry in patent application Ser. No. 542,639, filed Jan. 20, 1975 by the inventor of the instant application. Although the multivibrator of Ser. No. 542,639 was a versatile multivibrator circuit, capable of being turned on and off or synchronized by external pulses, and having its repetition rate controlled by an external voltage, the output waveshape of the above multivibrator was a highly linear voltage sweep. In other words, the voltage starts near zero, increases linearly with time, reaches a peak value, decreases almost instantaneously to its starting time, and then begins again. Although such a particular waveshape is useful for a number of purposes it fails to provide the exponential output waveshape necessary in instruments such as sweep generators, allowing an oscilloscope display of the frequency response of a device under test, with a logarithmic scale for the frequency axis.
Previous exponential sweep multivibrators used the exponential properties of a diode or transistor. Unfortunately, devices that display this property accurately over a wide range of inputs are not only extremely expensive to manufacture, but also produce an exponential property which varies significantly with temperature.